Add list of transferable badge types

Uber used to prevent very specific badge types from being transferred -
this changes that to a customizable list of badge types that CAN be
transferred.
Refactor is_transferrable property

The is_transferrable property was spelled incorrectly. It is not
is_transferable.
Add check for is_transferable to confirmation page

The attendee confirmation page now checks if the badge is transferable
before displaying the "Transfer" button.
Merge pull request #666 from magfest/remove_transferring_badges

Remove transferring badges